how gain impacts adc fsr, noise, and dynamic range

1 1

How gain impacts ADC FSR, noise, and dynamic range TIPL 4256 TI Precision Labs – ADCs

Created by Chris Hall & Bryan Lizon

Presented by Alex Smith

2

ADC full-scale range (FSR)

ADS124S08 FSR (24-bit delta-sigma ADC)

ADS8691 FSR (18-bit SAR ADC)

ADS8900B FSR (20-bit SAR ADC)

ADC w/ no integrated gain ADCs w/ integrated gain

3

ADS124S08 PGA input and output range

= ±0.75 V

= ±1.25 V

VREF

VSIG_OUT

FSROUT

= 2.5 V

= ±1.5 V

= ±2.5 V Gain = 2 V/V

𝐹𝑆𝑅𝐴𝐷𝑆124𝑆08 = ±𝑉𝑅𝐸𝐹

𝐺𝑎𝑖𝑛

60%

PGA

INPUT

PGA

OUTPUT

Refer both signal and

noise to the input

GAMP

VAINP

VAINN VOUTN

VOUTP

To access this calculator, navigate to the ADS124S08’s product folder on TI.com

VSIG_IN

FSRIN

4

Output- versus input-referred noise

VN,RTI is the system’s input resolution:

• If VIN < VN,RTI, the signal is below

the noise floor

• Else if VIN > VN,RTI, the signal can be

observed

For ADC w/ no gain, VN,RTO = VN,RTI = VN,ADC

Ideal ADC +

Equivalent ADC Noise Model:

VN,ADC

VN,RTO

VIN

5

Input-referred noise for amp + ADC

For ADC w/ gain, VN,RTO ≠ VN,RTI

ADC +

Equivalent Amp + ADC Noise Model:

VN,RTI

VN,RTO

VIN GAMP

Ideal ADC Ideal

Amplifier

𝑉𝑁,𝑅𝑇𝐼 = (𝑉𝑁,𝐴𝑀𝑃(𝑅𝑇𝐼))2 + 𝑉𝑁,𝐴𝐷𝐶/𝐺𝐴𝑀𝑃2

𝟎

GAMP*VN,AMP(RTI) >>VN,ADC

𝑉𝑁,𝑅𝑇𝑂 = (𝑉𝑁,𝐴𝑀𝑃(𝑅𝑇𝐼)∗ 𝐺𝐴𝑀𝑃)2 + 𝑉𝑁,𝐴𝐷𝐶2

6

Input-referred noise for 2x amps + ADC

𝑉𝑁,𝑅𝑇𝐼 = 𝑉𝑁,𝐴𝑀𝑃1(𝑅𝑇𝐼)2 +

𝑉𝑁,𝐴𝑀𝑃2(𝑅𝑇𝐼)

𝐺𝐴𝑀𝑃1

2

+𝑉𝑁,𝐴𝐷𝐶

𝐺𝐴𝑀𝑃1 ∗. 𝐺𝐴𝑀𝑃2

2

𝟎 𝟎

If GAMP1*GAMP2*VN,AMP1 (RTI) >> (GAMP2*VN,AMP2 (RTI))+VN,ADC, then VN,RTI = VN,AMP1(RTI)

Equivalent 2x Amplifier + ADC Noise Model:

+

VN,RTI

VIN VN,RTO

GAMP1

Ideal ADC Ideal

Amp 1

GAMP2

Ideal

Amp 2

ADC

7

Lower vs higher-resolution ADC total noise

𝑽𝑵,𝑮=𝟔𝟒

𝑽𝑵,𝑮=𝟏𝟐𝟖=

𝟏. 𝟐 µ𝑽𝑹𝑴𝑺

𝟎. 𝟔 µ𝑽𝑹𝑴𝑺= 𝟐

𝑽𝑵,𝑮=𝟔𝟒

𝑽𝑵,𝑮=𝟏𝟐𝟖=

𝟎. 𝟏 µ𝑽𝑹𝑴𝑺

𝟎. 𝟎𝟗 µ𝑽𝑹𝑴𝑺= 𝟏. 𝟏

Parameters

(Sinc 3, 60 SPS)

Gain Units

1 2 4 8 16 32 64 128

Noise, RTI 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089 µVRMS

Parameters

(Sinc 3, 60 SPS)

Gain Units

1 2 4 8 16 32 64 128

Noise, RTI 76.3 38.1 19.1 9.5 4.8 2.4 1.2 0.6 µVRMS

16-bit ADS114S08

24-bit ADS124S08

𝑽𝑵,𝑮=𝟏

𝑽𝑵,𝑮=𝟐=

𝟏. 𝟒 µ𝑽𝑹𝑴𝑺

𝟎. 𝟕 µ𝑽𝑹𝑴𝑺= 𝟐

𝑽𝑵,𝑮=𝟏

𝑽𝑵,𝑮=𝟐=

𝟕𝟔. 𝟑 µ𝑽𝑹𝑴𝑺

𝟑𝟖. 𝟏 µ𝑽𝑹𝑴𝑺= 𝟐

ADS114S08

ADS124S08

ADS1x4S08 block diagram

8

Applying the input-referred noise equation

𝑉𝑁,𝑅𝑇𝐼 = (𝑉𝑁,𝐴𝑀𝑃(𝑅𝑇𝐼))2 + 𝑉𝑁,𝐴𝐷𝐶/𝐺𝐴𝑀𝑃2

(𝑛𝑉𝑅𝑀𝑆)

GAMP*VN,AMP(RTI) < VN,ADC

Lower-resolution ADCs –

quantization noise dominates

GAMP*VN,AMP(RTI) >> VN,ADC

Higher-resolution ADCs –

thermal noise dominates

• Use a higher-noise (lower $) amp

• Larger gain if system allows

• Higher gain does not reduce noise

• Use a very low noise amp

9

How gain affects dynamic range (effective resolution)

Parameters

(Sinc 3, 60 SPS)

Gain

1 2 4 8 16 32 64 128

Full-scale range

(VREF = 2.5 V) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019

Noise, RTI

(µVRMS) 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089

Effective

resolution (bits) 21.8 21.8 21.7 21.5 21.3 20.4 19.5 18.7

Parameters

(Sinc 3, 60 SPS)

Gain

1 2 4 8 16 32 64 128

Full-scale range

(VREF = 2.5 V) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019

Noise, RTI

(µVRMS) 76.3 38.1 19.1 9.5 4.8 2.4 1.2 0.6

Effective

resolution (bits) 16 16 16 16 16 16 16 16

16-bit ADS114S08 24-bit ADS124S08

Increasing gain

Constant effective resolution

Increasing gain

Decreasing effective resolution

𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑟𝑎𝑛𝑔𝑒 (𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛) = 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆

𝑉𝑁,𝑅𝑀𝑆 (bits)

10

Dynamic range: max vs system (low resolution ADC)

7.5

8.5

9.5

10.5

11.5

12.5

13.5

14.5

15.5

16.5

1 2 4 8 16 32 64 128

Useful gain limit =

limited by FSR

Dyn

am

ic r

an

ge (

bit

s)

ADS114S08 (16-bit) dynamic range vs gain (VREF = 2.5V)

Gain (V/V)

𝑉𝐼𝑁 = 𝐹𝑆𝑅 = ±𝑉𝑅𝐸𝐹

𝐺𝑎𝑖𝑛

𝑉𝐼𝑁 = 10 𝑚𝑉

𝑆𝑦𝑠𝑡𝑒𝑚 𝐷𝑅

= 𝑙𝑜𝑔2𝑉𝐼𝑁,𝑅𝑀𝑆 (𝑅𝑇𝐼)

𝑉𝑁,𝑅𝑀𝑆 (bits)

𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐷𝑅

= 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆

𝑉𝑁,𝑅𝑀𝑆 (bits)

11

Dynamic range: max vs system (high resolution ADC)

13.5

14.5

15.5

16.5

17.5

18.5

19.5

20.5

21.5

22.5

1 2 4 8 16 32 64 128

Gain (V/V)

Dyn

am

ic r

an

ge (

bit

s)

Useful gain limit =

limited by amp noise

𝑉𝐼𝑁 = 𝐹𝑆𝑅 = ±𝑉𝑅𝐸𝐹

𝐺𝑎𝑖𝑛

𝑉𝐼𝑁 = 10 𝑚𝑉

𝑆𝑦𝑠𝑡𝑒𝑚 𝐷𝑅

= 𝑙𝑜𝑔2𝑉𝐼𝑁,𝑅𝑀𝑆 (𝑅𝑇𝐼)

𝑉𝑁,𝑅𝑀𝑆 (bits)

𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐷𝑅

= 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆

𝑉𝑁,𝑅𝑀𝑆 (bits)

ADS124S08 (24-bit) dynamic range vs gain (VREF = 2.5V)

12

How gain affects dynamic range (SNR)

ADS86x1 datasheet SNR values (dB)

FSR ADS8661

(12-bit)

ADS8671

(14-bit)

ADS8681

(16-bit)

ADS8691

(18-bit)

±3 * VREF 73.5 84.5 92 92.5

±2.5 * VREF 73.5 84.5 92 92.5

±1.5 * VREF 73.5 84.25 91.5 91.5

±1.25 * VREF 73.5 84.25 91.5 91.5

±0.625 * VREF 73.5 84 90 90

Increasing

gain

Constant

SNR

SNR

decreases

by 2 dB

SNR

decreases

by 2.5 dB

SNR

decreases

by 0.5 dB

ADS86x1 block diagram

Increasing resolution

𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑟𝑎𝑛𝑔𝑒 𝑆𝑁𝑅 = 20 ∗ 𝑙𝑜𝑔10𝐹𝑆𝑅𝑅𝑀𝑆

𝑉𝑁,𝑅𝑀𝑆 (dB)

13

Thanks for your time! Please try the quiz.

14

1. When is an external amplifier most effective at improving the system noise

performance?

a) For lower resolution devices

b) For higher resolution devices

c) Using an amplifier cannot improve noise performance.

Quiz: How gain impacts ADC FSR, noise & DR

15

Quiz: How gain impacts ADC FSR, noise & DR

1. When is an external amplifier most effective at improving the system noise

performance?

a) For lower resolution devices

b) For higher resolution devices

c) Using an amplifier cannot improve noise performance.

16

2. When will increasing the gain of an amplifier driving an ADC cause system

noise RTI to decrease?

a) When amplifier noise is the dominant noise source.

b) When ADC noise is the dominant noise source.

c) Increasing gain will always decrease system noise RTI

d) Increasing gain will never decrease system noise RTI

Quiz: How gain impacts ADC FSR, noise & DR

17

Quiz: How gain impacts ADC FSR, noise & DR

2. When will increasing the gain of an amplifier driving an ADC cause system

noise RTI to decrease?

a) When amplifier noise is the dominant noise source.

b) When ADC noise is the dominant noise source.

c) Increasing gain will always decrease system noise RTI

d) Increasing gain will never decrease system noise RTI

18

3. In the table below, the effective resolution is approximately the same for gains

of 1V/V to 16V/V. For gains of 32V/V and higher, the effective resolution

drops off quickly. Which of the following statements is not true.

a) For the low gain ranges the ADC noise is dominant, so the ratio of FSR and noise

remain the same.

b) For higher gain ranges the amplifier noise is dominant, so FSR decreases but noise

stays constant.

c) For higher gain ranges the ADC noise is dominant causing the effective resolution to

decrease.

Quiz: How gain impacts ADC FSR, noise & DR

Parameters

(Sinc 3, 60 SPS)

Gain

1 2 4 8 16 32 64 128

Full-scale range

(VREF = 2.5 V) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019

Noise, RTI

(µVRMS) 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089

Effective

resolution (bits) 21.8 21.8 21.7 21.5 21.3 20.4 19.5 18.7

𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆

𝑉𝑁,𝑅𝑀𝑆 (bits)

19

3. In the table below, the effective resolution is approximately the same for gains

of 1V/V to 16V/V. For gains of 32V/V and higher, the effective resolution

drops off quickly. Which of the following statements is not true.

a) For the low gain ranges the ADC noise is dominant, so the ratio of FSR and noise

remain the same.

b) For higher gain ranges the amplifier noise is dominant, so FSR decreases but noise

stays constant.

c) For higher gain ranges the ADC noise is dominant causing the effective resolution to

decrease.

Quiz: How gain impacts ADC FSR, noise & DR

Parameters

(Sinc 3, 60 SPS)

Gain

1 2 4 8 16 32 64 128

Full-scale range

(VREF = 2.5 V) ±2.5 ±1.25 ±0.625 ±0.313 ±0.156 ±0.078 ±0.039 ±0.019

Noise, RTI

(µVRMS) 1.4 0.7 0.37 0.21 0.12 0.11 0.1 0.089

Effective

resolution (bits) 21.8 21.8 21.7 21.5 21.3 20.4 19.5 18.7

𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑙𝑜𝑔2𝐹𝑆𝑅𝑅𝑀𝑆

𝑉𝑁,𝑅𝑀𝑆 (bits)